This invention relates to transducers for measuring motion and its direction, and in particular to a logic network for determining the direction of motion from the outputs of a position transducer.
The concept of providing electronic velocity feedback by generating a pulse train of fixed pulse width, using an optical tachometer or optical encoder, is well known. An optical encoder consists of a grid or scale of dark and transparent lines on a glass plate attached to a driven member, and two sections of corresponding grids on a stationary reticle. One section of reticle is spatially displaced from the other, and relative to the scale, one quarter of a line space so that when light is transmitted through the scale and the two reticles onto light sensors, the light detected by each sensor is modulated 90.degree. out of phase.
Quasi-sinusoidal output signals are produced by the light sensors with a frequency that is a direct function of the number of lines of the scale passing a fixed point per unit time. A pulse train may be derived from these quasi-sinusoidal output signals and converted to a velocity feedback signal of an amplitude linearly proportional to frequency.
The feedback voltage signal generated in such systems is proportional to the modulus of velocity and does not have any information as to the direction of motion. In many applications, the true direction of motion is crucial in order to apply the proper polarity of velocity feedback. What is required is a low cost system for determining from the quasi-sinusoidal signals the direction of any motion over less than one half the line spacing of the grids in the scale and reticles (typically 100 to 200 lines per inch).